The purpose of this research porposal is to study interactions of 1,25(OH)2D3 with other bone-active principles on osteoblast-like cells by examining (a) the influence of these factors on the biochemical responsiveness of the cells to 1,2(OH)2D3 and (b) their effects on the 1,24(OH)2D3 receptor system. This will be done in vitro using several lines of rat and human osteogenic sarcoma cells. We have determined that rat osteogenic sarcoma cells possess receptors for 1.25(OH)2D3 and that they respond to this hormone with increased synthesis and secretion of alkaline phosphatase activity; the cellular response to 1,25(OH)2D3 on alkaline phosphatase has also been observed in human osteogenic sarcoma cells. Further, we have detected glucocorticoid receptor in rat osteogenic sarcoma cells and have obtained evidence that glucocorticoids increase the concentration of the 1,25(OH)2D3 receptors. The concentration of the 1,25(OH)2D3 receptor appears to be regulated also by calcium. Using the osteogenic sarcoma cell model we shall examine the infulence of hormones such as glucocortocoids, PTH, and prostaglandins; ions such as calcium, phosphate, and magnesium; and ion-regulating factors such as A-23187, trifluoroperazine, and Verapamil on the biochemical responsiveness of the cells to 1,25(OH)2D3. In order to elucidate the mechanism of these interactions, we shall examine in parallel studies the effect of these factors as well as alkaline phosphatase, Lavamisol, and ATP on the regulation of the 1,25(OH)2D3 receptor system and determine the effects of calcium transport, calmodulin, and phosphorylation-dephosphorylation on the function of this receptor system. These studies should provide fundamental information about the hormonal and ionic regulation of bone cells and may provide insights regarding the physiology and pathophysiology of bone homeostasis.